It's because of the way it sits in the dock, but it does make it awkward charging when in handheld mode and tabletop mode the wiiu tablet used to have a charging point both on top and bottom of the gamepad which was good
If it’s not docked and in hand held mode the charging port is on the bottom so if you’re playing it not docked you have to buy a separate stand or something to be able to charge it.
you can very well charge it while playing, charge it while it's standing on its hinge? No not out of the box, but they make holding stands with chargers for that very situation.